Photomechanical methods have been widely used in producing printing original plates. These systems comprise: mask assembling and other steps (collectively referred to as "plate assembly") that are carried out from a camera-ready art drawing or a negative film of a photographed picture; preparing a prepress plate; and preparing proofs and a printing original plate (press plate) on the basis of the prepress plate, to thereby carry out printing. In recent years, on the other hand, a DTP (desk top publishing) process is occasionally practiced, in which all steps up to the preparation of a prepress plate are digitized. In the above-described DTP process, a prepress plate is prepared by an electronic output (such as exposure with a laser) from a computer having the necessary information such as text and graphic elements stored in a memory, and subsequent steps are done as in the photomechanical method to make proofs and a press plate, to thereby carry out printing. The DTP process has the advantage of eliminating the need for preparing a camera-ready copy for each proofing step and thereby simplifying the overall process. A more simplified approach called CTP (computer-to-plate) has been developed and it is characterized by carrying out all steps up to the production of a press plate by the digital imaging technology. With this method, not only proofing but also various image processing steps can be accomplished efficiently. In its most desirable situation, press plate making can be done directly without any special chemical and physical treatments.
Most of the substrates conventionally used to make press plates have layers of various kinds of light-sensitive materials provided on the surface thereof. They include silver halide salt based light-sensitive materials (silver salt photographic plates), diazo-based light-sensitive materials (presensitized or PS plates) and photoconductive materials (electrophotographic plates), and require various kinds of chemical and physical post-exposure treatments for effecting development and fixing. Press plates can also be made without any post-exposure treatments and a known method that meets this need is characterized by the provision of a silicone rubber based surface layer and the removal of a protective layer after exposure to enable waterless plate making. Both methods are commercialized extensively but suffer from the problem of process complexity; hence, a more efficient method has been desired. For details of these aspects, see, for example, "Insatsu Kogaku Binran (Handbook of Printing Technology)", edited by the Printing Society of Japan, published by Gijutsudo, 1987.
Two approaches, the electrophotographic transfer process (xerography) and the liquid ink-jet process, have recently been developed to produce direct press plates. In the former method, the toner image formed on a photoreceptor drum is transferred onto a substrate, thereby making a press plate in a convenient and high-speed way. However, because of the limitations in the construction of xerographic equipment, large plates (e.g., larger than A2 size) which are important in practical applications are difficult to make. In addition, the electrophotographic transfer process has a theoretical disadvantage in that fine toner particles will scatter in small quantities during the development and transfer steps to foul the background area, and this provides stops for ink deposition, often causing a problem in the actual printing operation.
On the other hand, the liquid ink-jet process is capable of producing large plates directly. However, if the solvent is water-based, the resin component generally remains highly hydrophilic even after deposition on the substrate, and this often causes a problem in the receptivity of an ink during printing. To deal with this problem, and also for preventing the spread of printing ink dots, the substrate for press plate making must be subjected to a special pretreatment. These problems are less noticeable if inks based on organic solvents are used. However, liquid ink-jet marking has the following theoretical difficulties: the need for a drying step; limitations in resin choice and deposited amount; and the short press life of the final plate. Many patent applications have been filed in the art of applications of the liquid ink-jet process to the production of prepress or press plates. Examples thereof include: JP-A-51-84303 (The term "JP-A" used herein means an "unexamined published Japanese patent application), JP-A-54-94901, JP-A-56-62157, JP-A-56-113456, JP-A-60-245587, JP-A-62-25081, JP-A-62-62157, JP-A-63-102936, JP-A-63-109052, JP-A-4-69244, JP-A-4-69245, JP-A-4-282249, JP-A-4-317065, JP-A-5-204138, JP-A-5-269958, JP-A-8-324145 and JP-B-58-8991 (The term "JP-B" used herein means an "examined Japanese patent publication").
To solve the problems in the conventional techniques, JP-B-64-27953 proposes a method and an apparatus for performing ink-jet recording using the solid ink, in which an image former which is prepared from natural waxes and the like and which are solid at ordinary temperatures (solid ink) is liquefied with heat, jetted against a substrate to be deposited on its surface and solidified, to thereby make a press plate. Since the ink is solvent-free, many of the solvent-related problems involved in liquid ink-jet processes are eliminated. In addition, natural waxes and the like are generally hydrophobic, so satisfactory ink receptivity is assured in the printing operation. In spite of these great benefits, the description in the publication is general and is not specific enough to allow for commercial production of the desired long-lived and reliable press plate unless more comprehensive experiments and modification efforts are made in many aspects including abrasion resistance, ink affinity, ease of printing and printing quality.
Ink dots made of a solid ink generally assume the shape of a hemispherical lens of a certain thickness when they are deposited on the substrate. This is advantageous for peeling a sheet of printing paper from the press plate having the printing ink deposited thereon. However, on the other hand, the deposited ink dots gradually wear and deform from the surface and the resulting change in diameter has been a major factor in shortening the press life. In addition, the above-described JP-A-64-27953 and other publications make no adequate discussion of the physical properties of the ink material and the affinity for ink and the performance on the press plate have often turned out to be extremely poor.